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As first reported by TVGuide.com, Adam Baldwin has been cast as a guest star in at least one upcoming episode of Castle season 4. Baldwin will be playing a character named Ethan Slaughter (seriously), a police detective who crosses path with Castle and Beckett.

Ethan Slaughter is no Kate Beckett. Instead, Slaughter is a rough-around-the-edges member of New York City's gang unit who comes into contact with the Castle crew in the course of a case.

Castle, understandably, is fascinated with this extreme character. But when the curious writer gets the chance to shadow the rough and violent Slaughter on a case, Castle is forced to reconsider exactly what he is willing to do in the name of justice.

There is no word yet as to when exactly Adam Baldwin will appear on Castle, nor do we know if Baldwin will guest-star in more than one episode. He will, however, arrive toward the end of the season -- Slaughter's actions help place Castle in a dangerous position for the cliffhanger coming at the end of Castle season 4.

Nathan Fillion and Adam Baldwin were co-stars in the short-lived Joss Whedon show, Firefly, which aired for less than one season in 2002 (yet still inspired a cult following). Baldwin played the mercenary Jayne Cobb, a crewmember on the spaceship commanded by Fillion's Captain Malcolm Reynolds. The two actors worked together again in 2005's Serenity (a feature-film conclusion to the Firefly series). Since then, Adam Baldwin's co-starring role on Chuck has kept him from reuniting with his former castmate. The Castle role will be Baldwin's first TV gig since Chuck concluded its run in January of this year.

Does the appearance of Adam Baldwin bode well for the appearance of other Firefly stars on Castle? How will Beckett react to a police-department rival for Castle's attentions? What sort of mischief can we expect from the pairing of Castle and Slaughter? Let us know what you think in the comments section below!